This invention relates to two-way radios in general, and in particular to improving squelch in narrow band carrier frequency radios.
Two-way radios use what is known as squelch mode in which the radio speaker is silenced when there is no signal being received on an available radio frequency carrier. In large crowded cities available radio frequency communication channels, or carrier frequencies, will be affected by noise from both electrical equipment and cross channel interference. Accordingly, this noise may result in two-way radios that use squelch mode intermittently going to unsquelch mode. This can be annoying to users of such two-way radios and to overcome this problem wider squelch bandwidths are often used. Therefore unsquelching will only occur when a relatively strong carrier signal is present. However this solution is only satisfactory in urban areas where such relatively strong carrier signals are usually present.
Another problem associated with conventional squelch circuits is that undesirable clamping may result in which the two-way radio will intermittently go to squelch mode if high frequency voice signals fall within the squelch detector bandwidth.
It is an aim of this invention to overcome or alleviate at least one of the problems associated with two-way radios using squelch or at least provide the public with a useful alternative.
According to one aspect of the invention there is provided a two-way radio including:
a radio frequency receiver having a modulated radio frequency signal output;
a variable bandwidth filter having an input coupled to said modulated radio frequency signal output;
demodulator circuitry coupled to said variable bandwidth filter;
an audio transmitter; and
processing circuitry having an input coupled to said demodulator circuitry and an output coupled to said audio transmitter, said processing circuitry having a control output coupled to a control input of said variable bandwidth filter, said processing circuitry providing for a squelch and unsquelch mode,
wherein in use when said radio is operating in said squelch mode, said processing circuitry controls said variable bandwidth filter to have a first bandwidth and when said radio is operating in said unsquelch mode said processing circuitry controls said variable bandwidth filter to have a second bandwidth which has a smaller frequency range than said first bandwidth.
Preferably, said radio may be characterised by said first bandwidth having a frequency range which is at least 20% greater that of said second bandwidth.
Suitably, the second frequency range may be within the first frequency range.
Suitably, said processing circuitry may include a microprocessor and signal processor.
Preferably, said variable bandwidth filter and said demodulator circuitry may be in a single integrated circuit.
Suitably, said radio may also include:
a frequency synthesizer controllable by said microprocessor; and
a mixer having a first input coupled to said a modulated radio frequency signal output, and a second input coupled to an output of said frequency synthesizer, said mixer providing an output intermediate frequency to said variable bandwidth filter.
Suitably, said audio transmitter may be a speaker.
Preferably, said demodulator circuitry may be adapted for demodulating an amplitude modulated signal. In an alternative form, said demodulator circuitry may be adapted for demodulating a frequency modulated signal.